In the processing of hydrocarbons, alumina supports are widely used due to theirs having relatively large specific surface areas and good stabilities. A useful application is the use as supports of selective hydrogenation catalysts for pyrolysis gasoline.
Pyrolysis gasoline is a by-product of ethylene industry. When producing ethylene by steam cracking from liquid feed such as naphtha, gas oil, and the like, an output of pyrolysis gasoline may be about 60 wt. % or more of the throughput of ethylene. Pyrolysis gasoline is typically subjected at first to a first-stage hydrogenation to remove highly unsaturated olefins, such as diene and styrene, and then to a second-stage hydrogenation to remove monoolefin as well as impurities containing sulfur, nitrogen, oxygen, and the like. The hydrotreated pyrolysis gasoline is then mainly used for producing aromatic hydrocarbons. Catalysts currently used in industry for the selective hydrogenation of pyrolysis gasoline are mainly Pd-based catalysts or Ni-based catalysts. During a selective hydrogenation reaction of pyrolysis gasoline, in particular full boiling range pyrolysis gasoline, due to the presence of impurities and poisons, such as unsaturated components, gums (i.e., polymers resulted from the polymerization of unsaturated components, such as dienes and styrene), heavy components, free water, heavy metals, etc. in the feed to be treated while industrial plants lack generally effective pretreating means, the introduction of such a feed directly to a catalyst bed inside a reactor may result in: (1) the catalyst will easily coke; (2) activity of the catalyst is reduced or lost because the Al2O3 support of the catalyst itself is hydrophilic, and thus preferentially adsorbs and cumulates water so as to make the catalyst oleophobic so that the adsorption of hydrocarbon reactants on the catalyst is cumbered; (3) activity of the catalyst is reduced or lost because the heavy components, oligomer gums, and the like are adsorbed onto the surface of the catalyst so as to block adsorbing-diffusing-desorbing channels so that the adsorption of reactants onto the catalyst is cumbered; (4) activity of the catalyst is reduced or lost because of the binding of poisons, such as heavy metals, arsenic, etc., in the feed to active sites of the catalyst by strong coordination; (5) light components which are more easily hydrogenated will unduly react so as to cause loss in yield of aromatic hydrocarbons, alternatively, heavy components which are more difficulty hydrogenated will incompletely react so as to render product unqualified. These will result in that the quality of the operation of the catalyst is reduced and that the cycle length and service lifetime of the catalyst are shortened, so that the catalyst has to be activated and regenerated frequently. Catalysts having relatively large pore volumes and relatively large pore diameters may reduce the buildup of gums and free water on the catalysts, and thus are more tolerant to the gums and the free water in the feed and capable of delaying the deactivation of the catalysts, thereby prolonging the cycle length and service lifetime of the catalysts. A pore structure of a supported catalyst is determined by a support forming the catalyst, and the preparation of a support having a relatively large pore volume and a relatively large pore diameter while ensuring that the catalyst has good activity at lower temperature therefore is a key for the preparation of a selective hydrogenation catalyst for pyrolysis gasoline, in particular, a hydrogenation catalyst useful in the hydrogenation of a full boiling range pyrolysis gasoline having a higher gum content and a higher free water content.
The introduction of pore enlarging agent during the colloid formation or washing in the preparation of pseudoboehmite, alternatively, during the moulding of pseudoboehmite is a process commonly used in the prior art for preparing an alumina support having larger pore volume and larger pore diameter.
CN1714937A discloses a process for preparing a molded alumina support having a large pore volume, comprising mixing aluminum ammonium carbonate and a nitrogen-containing compound other than acids, moulding and calcining at 350 to 650° C. for 1 to 8 hours. The support obtained by this process has a specific surface area of 200 to 350 m2/g, and a mean pore diameter of 25 to 35 nm.
CN1689703A discloses a process for preparing an alumina support with dual apertures, comprising mixing an alumina hydrate with a nitrogen-containing compound other than acids and a halide, moulding and calcining at 600 to 850° C. for 1 to 10 hours. The obtained support has such a pore distribution that a pore volume of pores having a pore diameter of 10 to 20 nm comprises 35 to 80% of the total pore volume, a pore volume of pores having a pore diameter of 500 to 1200 nm comprises 15 to 60% of the total pore volume, and sum of pore volumes of pores having a pore diameter of less than 10 nm, of pores having a pore diameter of from larger than 20 nm to less than 500 nm, and of pores having a pore diameter of larger than 1200 nm comprises 5 to 40% of the total pore volume. The obtained support has an acid amount of 0.05 to 0.2 mmol/g.
CN1647857A discloses a process for preparing a macroporous alumina support, comprising mixing a pseudoboehmite having a crystallinity of less than 70% and an organic pore enlarging agent, moulding the mixture and calcining the moldings at 600 to 1100° C. for 0.5 to 4 hours. The support obtained by this process has a pore volume of 0.9 to 1.3 cm3/g, and pores having a pore diameter of larger than 100 nm comprises 7 to 25% of the total pore volume.
CN1600430A discloses a process for preparing a macroporous alumina support, comprising mixing an alumina hydrate with a pore enlarging agent, moulding and calcining at 600 to 850° C. for 1 to 10 hours. The obtained support has a probable pore diameter of 14 to 20 nm, a pore volume of 0.6 to 1.2 cm3/g, a specific surface area of 150 to 200 m2/g, and an acid amount of 0.05 to 0.2 mmol/g.
Thus it can be seen that, although some supports having relatively large pore volumes can be prepared by using a pore enlarging agent, such supports contain a considerable amount of micropores less than 10 nm in pore diameter, and the pore distribution is relatively broad. Furthermore, if the pore enlarging agent is used in a smaller amount, its effect might be inconspicuous, whereas if the pore enlarging agent is used in a larger amount, it might adversely impact properties of the support, such as mechanical strength.
Chinese Patent Application CN1635054A discloses a catalyst used for the selective hydrogenation of heavy distillations of pyrolysis gasoline as well as preparation and use thereof. The alumina support contains 1 to 3 wt. % of an alkaline earth metal or an oxide thereof disposed on its surface, and contains Pd and Mo, alternatively, Pd and W, as active components, wherein the content of Pd is in a range of from 0.24 to 0.35 wt. %, and weight ratio of Pd to Mo or Pd to W is in a range of from 1:0.5 to 1:2.5. It is said that the catalyst can be used in the hydrogenation of C5 to C9 distillations, in particular, C8 to C9 heavy distillations of pyrolysis gasoline, and exhibits a high activity at lower temperature, a high tolerance to As, S, O, or N impurity, a high tolerance to gums, and a stable activity. However, the patent application is silent about the tolerance of the catalyst to water.
Chinese Patent CN1184289C discloses a catalyst used for the selective hydrogenation of pyrolysis gasoline as well as preparation and use thereof. The catalyst comprises titania-alumina complex as a support and metal Pa as an active component supported on the complexed support, wherein the content of metal Pa is 0.25 to 0.35 wt. %, based on the total weight of the catalyst. It is said that the catalyst can be operated under high space velocity of feed and has good selectivity and stability. However, the patent is silent about the tolerance of the catalyst to water and gums.
Chinese Patent Application CN1181165A discloses a selective hydrogenation catalyst consisting of 0.15 to 0.5 wt. % of Pd, 0.1 to 3.0 wt. % of an oxide of an alkaline earth metal, and alumina support, wherein the alumina support has a specific surface area of 50 to 150 m2/g and a pore volume of 0.35 to 0.55 cm3/g, and pores having a pore radius of 5.0 to 10.0 nm provide more than 70% of the total pore volume. It is said that the catalyst is suitable for not only a first-stage selective hydrogenation of pyrolysis gasoline but also a process of selectively hydrogenating highly unsaturated hydrocarbons in C3 to C6 distillate oil to monoolefins. However, the catalyst has a relatively high active temperature, and the patent application is also silent about the tolerance of the catalyst to water and gums.
There therefore remains a need for a selective hydrogenation catalyst which can be operated under a higher space velocity of a feed and has a higher activity at lower temperature, a good selectivity, and a good tolerance to water and gums, to prolong regeneration cycle length and service lifetime of the catalyst.